REDHAT-BUG-2380949: High severity Nlnet Labs Unbound vulnerability
A multi-vendor cache poisoning vulnerability named 'Rebirthday Attack' has been discovered in caching resolvers that support EDNS Client Subnet (ECS). Unbound is also vulnerable when compiled with ECS support, i.e., '--enable-subnet', AND configured to send ECS information along with queries to upstream name servers, i.e., at least one of the 'send-client-subnet', 'client-subnet-zone' or 'client-subnet-always-forward' options is used. Resolvers supporting ECS need to segregate outgoing queries to accommodate for different outgoing ECS information. This re-opens up resolvers to a birthday paradox attack (Rebirthday Attack) that tries to match the DNS transaction ID in order to cache non-ECS poisonous replies.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Configuration
Rebuild Unbound without ECS support by not compiling with '--enable-subnet' (to avoid the Rebirdtable Attack when ECS is used).
Unbound --enable-subnet (ECS support at compile time) = disabled - Configuration
If Unbound is compiled with ECS support ('--enable-subnet'), configure it so it does not send ECS to upstream resolvers by ensuring none of these options are used: 'send-client-subnet', 'client-subnet-zone', or 'client-subnet-always-forward'.
Unbound ECS forwarding options (send-client-subnet / client-subnet-zone / client-subnet-always-forward) = do not send ECS to upstream - Compensating control
For caching resolvers that support EDNS Client Subnet (ECS), segregate outgoing queries to accommodate different outgoing ECS information so that non-ECS poisoned replies are not cached for ECS-disabled or differing ECS contexts.
Event History
Frequently Asked Questions
What is the severity of REDHAT-BUG-2380949?
The severity of REDHAT-BUG-2380949 is classified as high due to the potential for cache poisoning attacks.
How do I fix REDHAT-BUG-2380949?
To fix REDHAT-BUG-2380949, upgrade to the latest version of Unbound that addresses this vulnerability.
Which versions of Unbound are affected by REDHAT-BUG-2380949?
Unbound compiled with EDNS Client Subnet support and configured to send ECS information is vulnerable to REDHAT-BUG-2380949.
What systems are impacted by REDHAT-BUG-2380949?
Systems running Unbound with ECS enabled and configured to send ECS data in DNS queries are impacted by REDHAT-BUG-2380949.
Is there a workaround for REDHAT-BUG-2380949?
A temporary workaround for REDHAT-BUG-2380949 is to disable ECS support in Unbound until a patch is applied.